In certain applications, components must be manufactured from large diameter metal or metal alloy preforms which are substantially free of defects. (For ease of reference, the term “metallic material” is used herein to refer collectively to unalloyed metals and to metal alloys.) One known method for producing high quality preforms is spray forming, which is generally described in, for example, U.S. Pat. Nos. 5,325,906 and 5,348,566. Spray forming is essentially a “moldless” process using gas atomization to create a spray of droplets of liquid metal from a stream of molten metal. Spray forming, however, suffers from a number of disadvantages that make its application to the formation of large diameter preforms problematic. Furthermore, an unavoidable byproduct of spray forming is overspray, wherein a portion of the metal spray misses the developing preform altogether or solidifies in flight without attaching to the preform. Average yield losses due to overspray in spray forming can be 20-30%.
Another method for producing high quality preforms is nucleated casting, which is generally described in, for example, U.S. Pat. Nos. 6,496,529 and 7,154,932. Nucleated casting is essentially a process involving using gas atomization to create a spray of droplets of liquid metal and depositing the droplet spray into a mold. In various circumstances, portions of the droplet spray, i.e., the overspray, may accumulate on a top surface of the mold. In some instances, the overspray accumulated on the mold's top surface bonds with a preform being cast within the mold. In these circumstances, the nucleated casting process may have to be stopped in order to remove the overspray, and this may result in scrapping the preform. Accordingly, there are drawbacks associated with certain known techniques in which preforms are cast from a droplet spray. Thus, a need exists for an improved apparatus and method for nucleated casting of metallic materials.